Electric heating – Metal heating – By arc
Reexamination Certificate
2000-07-11
2002-03-05
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121430, C118S7230IR, C315S111410
Reexamination Certificate
active
06353201
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the form of a discharge electrode and a power supply method employed by an RF plasma generation apparatus for use in forming semiconductor films of amorphous silicon, microcrystalline silicon, polycrystalline silicon, silicon nitride, etc. to be used in solar cells, thin-film transistors, etc. as well as for use in etching such semiconductor films.
2. Description of the Related Art
As examples of the RF plasma generation apparatus, there will be described two structures used in a plasma-enhanced chemical vapor deposition apparatus (hereinafter called “PCVD” or a “vapor deposition apparatus”) used conventionally to form thin films of amorphous silicon (hereinafter called “a-Si”) and thin films of silicon nitride (hereinafter called “SiNx”); i.e., {circle around (1)} a structure using a ladder electrode for exciting discharge; and {circle around (2)} a structure using a parallel-plate electrode for exciting discharge.
{circle around (1)} First, the structure using a ladder electrode is described. Japanese Patent Application Laid-Open (kokai) No. 236781/1992 discloses a plasma-enhanced CVD apparatus which uses a ladder-like flat coil electrode assuming any of various shapes. A typical example of this strucutre will be described with reference to FIG.
21
.
As shown in
FIG. 21
, in this PCVD apparatus, a discharge-exciting ladder electrode (may hereinafter be called a “ladder electrodes”)
02
and a substrate heater
03
are arranged in parallel with each other within a reaction chamber
01
. An RF power of, for example, 13.56 MHz is supplied to the discharge-exciting ladder electrode
02
from an RF power source
04
via an impedance-matching unit
05
.
As shown in
FIG. 22
showing a perspective view of the discharge-exciting ladder electrode
02
, the RF power source
04
is connected to one end of the ladder electrode
02
via the impedance-matching unit
05
, whereas a grounding line
06
is connected to the other end of the ladder electrode
02
, whereby the ladder electrode
02
is grounded, together with the reaction chamber
01
shown in FIG.
21
.
RF power supplied to the discharge-exciting ladder electrode
02
causes generation of glow discharge plasma between the substrate heater
03
and the discharge-exciting ladder electrode
02
, which are disposed within the reaction chamber
01
. Then, the supplied RF power flows to the ground through the grounding line
06
of the discharge-exciting ladder electrode
02
. A coaxial cable is used as the grounding line
06
.
A reaction gas
08
; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber
01
from unillustrated cylinders through a reaction gas feed pipe
07
. The supplied reaction gas
08
is decomposed by glow discharge plasma generated by the discharge-exciting ladder electrode
02
. The resulting substance is deposited on a substrate
09
, which is held on the substrate heater
03
and is heated to a predetermined temperature. The gas within the reaction chamber
01
is evacuated therefrom through an evacuation pipe
010
and by means of a vacuum pump
011
.
Next will be described formation of a thin film on a substrate effected by use of the above-described apparatus. As shown in
FIG. 21
, the vacuum pump
011
is driven so as to evacuate the reaction chamber
01
. Subsequently, the reaction gas
08
; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber
01
through the reaction gas feed pipe
07
so as to maintain the pressure within the reaction chamber
01
at 0.05 to 0.5 Torr.
In this state, RF power is applied to the discharge-exciting ladder electrode
02
from the RF power source
04
to thereby generate glow discharge plasma. The reaction gas
08
is decomposed by glow discharge plasma generated between the discharge-exciting ladder electrode
02
and the substrate heater
03
. As a result, radicals including Si, such as SiH
3
and SiH
2
, are generated and adhere to the surface of the substrate
09
, thereby forming an a-Si thin film.
{circle around (2)} Next, the structure using a parallel-plate electrode for exciting discharge will be described with reference to FIG.
23
.
As shown in
FIG. 23
, an RF electrode
022
and a substrate heater
023
are arranged in parallel with each other within a reaction chamber
021
. An RF power of, for example, 13.56 MHz is supplied to the RF electrode
022
from an RF power source
024
via an impedance-matching unit
025
. The substrate heater
023
, together with the reaction chamber
021
, is grounded, thereby serving as a grounding electrode. Accordingly, glow discharge plasma is generated between the RF electrode
022
and the substrate heater
023
.
A reaction gas
027
; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber
021
from unillustrated cylinders through a reaction gas feed pipe
026
. The gas within the reaction chamber
021
is evacuated therefrom through an evacuation pipe
028
and by means of a vacuum pump
029
. A substrate
030
is held on the substrate heater
023
and is heated to a predetermined temperature.
Through use of the thus-configured apparatus, a thin film is formed in the following manner. As shown in
FIG. 23
, the vacuum pump
029
is driven so as to evacuate the reaction chamber
021
. Next, the reaction gas
027
; for example, a mixed gas of monosilane and hydrogen, is supplied to the reaction chamber
021
through the reaction gas feed pipe
026
so as to maintain the pressure within the reaction chamber
021
at 0.05 to 0.5 Torr. A voltage is applied to the RF electrode
022
from the RF power source
023
to thereby generate glow discharge plasma.
Monosilane gas contained in the reaction gas
027
supplied through the reaction gas feed pipe
026
is decomposed by glow discharge plasma generated between the RF electrode
022
and the substrate heater
023
. As a result, radicals including Si, such as SiH
3
and SiH
2
, are generated and adhere to the surface of the substrate
030
, thereby forming an a-Si thin film.
However, the conventional structures {circle around (1)} and {circle around (2)} using a ladder electrode and a parallel-plate electrode, respectively, for exciting discharge involve the following problems:
{circle around (1)} An electric field generated in the vicinity of the ladder electrode
02
shown in
FIG. 21
causes decomposition of the reaction gas (for example, SiH
4
)
08
into Si, SiH, SiH
2
, SiH
3
, H, H
2
, etc., thereby forming an a-Si film on the surface of the substrate
09
. However, when the frequency of the RF power source is increased from current 13.56 MHz to a frequency of 30 MHz to 300 MHz (very high frequency band (hereinafter called the VHF band)) in order to increase the film deposition rate in formation of the a-Si film, uniformity of electric-field distribution in the vicinity of the ladder electrode
02
is impaired, resulting in a significant impairment in thickness distribution of the a-Si film formed on the substrate
09
.
FIG. 24
shows the relationship between plasma power-source frequency and film-thickness distribution (deviation from an average film thickness) in the case of film deposition on a substrate having an area of 30 cm×30 cm effected by use of the ladder electrode
02
. Uniformity (within ±10%) of film-thickness distribution can be reliably maintained for a substrate size, or substrate area, of about 5 cm×5 cm to 20 cm×20 cm.
The structure using the ladder electrode
02
encounters difficulty in forming a uniform film through employment of the VHF band, for the following reason. As shown in Table 1, the wavelength of the VHF band ranges from 1 m to 10 m in vacuum, showing an order equivalent to that of the circuit size of the film-forming apparatus. The wavelength is shortened further in a distributed-constant line, such as the coaxial cable used to transmit power or the ladder electrode. In the case of the coaxial cable, the wavelength is shortene
Hisatome Masatoshi
Horioka Tatsuji
Kokaji Satoshi
Mashima Hiroshi
Morita Shoji
Armstrong Westerman & Hattori, LLP
Mitsubishi Heavy Industries Ltd.
Paschall Mark
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